1. Field of the Invention
The present invention relates to zoom lenses and image pickup apparatuses having the same, and more in particular to zoom lenses suitable for use in image pickup apparatuses, such as video cameras, silver-halide film cameras, digital cameras, television cameras, and surveillance cameras.
2. Description of the Related Art
In recent years, imaging optical systems for image pickup apparatuses such as video cameras and digital still cameras are required to have a wide field angle, a high zoom ratio, and high optical performance over the entire zoom range. To meet such requirements, a zoom lens composed of four groups of lenses including, in order from the object side to the image side, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power has been proposed. The side of a lens where an object to be imaged is placed is referred to as the object side or front side of the lens; and the side of the lens where the image is formed is referred to as the image side or back side of the lens.
For example, U.S. Pat. Nos.: 5,963,378, 6,166,864, and 7,193,787 disclose rear-focusing type four-group zoom lenses, in which the second lens group is moved to perform zooming, and the fourth lens group performs focusing and is moved to correct image plane variation caused by zooming. Other examples that include U.S. patent application publication No. 2008/0043344 and U.S. Pat. No. 7,466,496 disclose four-group zoom lenses having a high zoom ratio of about 20×.
In general, in order to increase the zoom ratio, the refractive power of a lens group for zooming is increased and the traveling distance for zooming is increased. However, in such a zoom lens, the aberration variation occurring during zooming is large, which makes it difficult for the zoom lens to achieve high optical performance over the entire zoom range.
Therefore, in order to achieve high optical performance over the entire zoom range while increasing the field angle and zoom ratio, it is important to appropriately set the zoom type, the refractive power of each lens group, and the lens configuration of each lens group. In particular, in the above-mentioned rear-focusing type four-group zoom lenses, it is important to appropriately set the configuration of the second lens group, which mainly performs zooming.